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Arsenic Mapping for North 24- Pargana District of West Bengal –using GIS and Remote Sensing technology
Balen Basu, PCI Geomatics
Technical Manager
Email: basu@pcigeomatics.com
Samik Sil
Sr. Product Executive
Email: samik_sil@yahoo.com
PCI Software Pvt. Ltd.,CB-55,Salt Lake, Calcutta-700064
Ph-033-2337-3172,Fax- 033-2359-8071
Abstract
The healthy living of citizens depends upon the successful and optimal exploitation of the natural resources, however a slight imbalance in any equilibrium is bound to manifest itself in the form of what we call as environmental hazard. Understanding the natural occurrence of any hazardous materials, its chemical and mineral forms in the surface environment is of paramount importance in assessing the sources and pathways contributing to human exposure. Arsenic is one such element within Group V b of the Periodic Table, but is often incorrectly referred to as a metal. It is ubiquitous in the environment, usually being present in small amounts in all rocks, soil, dust, water, air and biological tissues. Though the various effects of arsenic on human body is understood, but the knowledge of its source, hydro geochemistry and pathway is still under research and study.
The present project was aimed at mapping the arsenic presence, the contour of its concentration and the depth of its presence in the district of North 24 Parganas in West Bengal, India. For the purpose, Geographical Information system and Satellite Image Processing was used to identify, locate, map and analyze the existing data on the district for modeling the hazard zones in the district. Though arsenic mapping requires a broad zonal understanding of the whole dynamics to achieve a strategic mapping and remediation, yet the present study at block level for a single district can act as a precursor to the whole process to initiate and set a strategic model subsequently. Satellite image IRS 1C ( LISS) has been used along with the block map from district and planning series of NATMO, for preparing the base map. This project is an unique example of applying GIS and Remote Sensing for environmental mapping.
Introduction
Groundwater is one of the most important sources of drinking water and is not presence in abundance in nature. The contamination of groundwater with Arsenic is one of the serious problems encountered in developing countries. Thus there is a rising need to map the level of arsenic concentration, trend of arsenic flow and the temporal changes that occur in concentration level. The present study was an effort to map arsenic concentration in a district level that might lead to a broader understanding of it’s regional presence and significance.
India and Bangladesh has been reported to suffer a lot from Arsenci contamination problems. West Bengal is one of the affected states in India. According to a UNICEF study, access to safe drinking water in Bangladesh has declined by 17% in the last three years due to arsenic contamination. The most commonly observed symptoms of chronic arsenic poisoning are conjunctivitis, melanosis and hyperkeratosis. In severe cases, gangrene in the limbs and malignant neoplasm have also been observed.
The geochemistry of arsenic had been recently been reviewed by Thornton (1996). The main constituent of around 200 mineral species is arsenic. Out of these 60% are arsenates, 20% sulphides and sulphosalts and the remaining 20% includes arsenides, oxides, silicates. From the observations in the Cordoba it was concluded (Astolfi et al., 1981) that the regular intake of drinking water containing more than 0.1 mg l-1 of arsenic leads to clearly recognizable signs of arsenic toxicity and ultimately in some cases to skin cancer.
The solution to such a problem lies in understanding the intricate relation between the various socio-economic factors associated with it. It has been found that Arsenic consumed may be quickly excreted from the body through methylation (often termed as detoxification) in the body through mostly urine. But this methylation reaction needs methyl donors coming from a balanced nutritional methionine-rich food sources like green vegetation and meat. Thus economic conditon, the demographic status is also a key to such solution.The geology of the area, the land use and the irrigation and drainage pattern attributes to finding a plausible solution for such problem.
Geographical Information System acts as an excellent tool to unify data from various source and integrate them into a single environment to analyse the relationship amongst them. The satellite images helps in identifying the various land use pattern and may provide a clue to identification of patterns and source with respect to it’s geological setup. Thus Geomatics can act as a decision support tool to analyse the various data source for mapping the risk zone map of the area.
The present project was mooted by PHED and a pilot project was undertaken using PCI Technology ( through industry participation of Lord’s Infotech) using the various groundwater data available in the department for mapping the arsenic concentration in the district. The other available data on Chlorine, Iron contents and depth of occurrence were also used for preparing a hazard zone map of the area and site suitability map for safe drinking water zones of the area. The latest satellite images were used to detect the various land use and land cover of the district to verify the changing pattern of surface water presence of the area. An attempt has been made to model the affected zone map using various GIS technology.
Regional Setting :
The district of North 24 Parganas of West Bengal is in the southern part of the Bengal Basin. The geographical extent of the district lies between 88d19m E, 23d20m N to 89d10m E,22d01m N . The basin is actually a peri-cratonic basin and comprises of Ganga-Brahmaputra delta in the southern part. It had broken from the Gondowanaland along the margin of the Indian plate and then moved towards northerly in the early Cretaceous (125Myr ago). The collision of the Indian and European plate began in the early Eocene ( 40 –41 Myr ago) and resulted in the Himalayas. Due to this the two sediments from Ganga and Brahmaputra got subsequently merged. Relatively recent folding and uplift (Quaternary epoch) of the Brahmaputra sediments close to the intraplate boundary redirected the course of the Brahmaputra to its present configuration (Morgan et al., 1959; Lindsay et al.,1991).
There is a thickening of the Ganga-Brahmaputra delta towards the south and has three stratigraphic sequence – the proto-Ganges delta, the transitional delta and the modern delta( 11 Myr ago) with a successive sequence of sands, sandy mud, silt and mud which were deposited under a major eustatic sea level low at about 11 Myr ago. The modern delta has been formed primarily of alluvial sediments transported by the rivers, e.g. the Mayurakshi, the Ajoy, the Damodar etc. originating from the Chotanagpur Uplands in the west and subsequently by the rivers flowing from the Himalayan foredeep basin e.g. the Ganges, the Padma, the Bhagirathi, the Brahmaputra etc. flowing from the north when a gap, the Garo-Rajmahal gap, was created due to tectonic movements (Auden, 1949). Arsenic contaminated groundwater occurs in the modern deltaic sediments.
From the IRS LISS satellite images certain geomorphological features are distinctively present. The delta can be divided into two regions: (1) the upper delta plain of meander belts of the Padma- Bhagirathi rivers in the north; and (2) the lower delta plain with several tidal creeks in the south. The upper delta plain is characterised by a series of meander scars of various wavelengths and amplitudes, abandoned channels, oxbow lakes, formed under varying hydrodynamic conditions in a fluvial regime. Abandoned meander scrolls are the most common form and could be related to flood-plain formation in the upper delta plain with a very gentle southerly slope.
Earlier report on the hydrogeology of the area suggests that there are shallow aquifer (12 - 15 m below ground level, bgl) in the upper delta plain and is mostly under unconfined conditions except near its southern fringe where it occurs under semi-confined to confined conditions. There are two more aquifers with depths ranging from 35 to 46 m and 70 to 150 m in the districts North 24-Parganas. There is generally a southeasterly gradient of the water surface sub-parallel to the general slope of the area. All the aquifers are interconnected due to spatial variations in grain size.
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